In all-trans-neurosporene, whose 1B(u)(-)(0) level is lower than the 1B(u)(+)(0) level by one vibrational quantum, the following two steps of vibrational relaxation from the mixed vibronic states were identified, in the order, 1B(u)(+)(1) + 1B(u)(-)(2) stimulated emission -> 1B(u)(+)(0) + 1B(u)(-)(1) stimulated emission -> 1B(u)(-)(0) transient absorption. The stimulated emission patterns were simulated by the use of Franck-Condon factors in the transitions from the pair of mixed vibronic levels down to the ground 1A(g)(-)(v '') vibronic levels. The vibronic levels of the mixed 1B(u)(+) and 1B(u)(-) state were characterized theoretically based on the diabatic approximation. (c) 2007 Elsevier B.V. All rights reserved.